Well it's certainly a tree and does look like a sucker. Possibly a Prunus but I wouldn't like to be quoted on that. What trees are growing nearby? (20 feet doesn't preclude suckers)

Funny that you mention Prunus. The tree growing 20 feet away is a weeping cherry tree; however, the leaf size on the unidentified plant is much larger.

The weeping cherry is grafted onto a sweet cherry which has produced a root sucker.